This application claims the benefit of U.S. Provisional Application No. 60/003,919, filed Sep. 18, 1995, entitled LIDAR ATMOSPHERIC WIND DETECTOR, and of U.S. Provisional Application No. 60/003,926, filed Sep. 18, 1995, entitled HOLOGRAPHIC OPTICAL ELEMENT BEAM DIRECTOR FOR LIDAR.
1. The Field of the Invention
The present invention pertains to beam directors in the field of wind sensing using lidar, and particularly the use of lidar in detecting the speed and direction of objects such as air-born particles and molecules in the atmosphere in order to determine the speed and direction of the wind which is carrying them.
2. The Background Art
The advance of new and innovative applications for lasers has pushed forward the development of technologies which support those applications. Atmospheric research using laser technologies in particular has been a growing field among the ever increasing applications of laser light. For purposes of convenience, the term "light" will be used to mean electromagnetic radiation of all frequencies and wavelengths. It has been found that laser light can be used to make both precise and accurate measurements, which is particularly useful in determining atmospheric wind characteristics such as wind speed and direction. Wind speed and direction are important to a number of disciplines, including the scientific fields such as meteorology and atmospheric research as well as applied fields such as military and commercial travel. These disciplines, and others, have demonstrated an ever increasing need for accurate, precise and cost effective information regarding wind; however, such acquisition of information has presented a number of challenges. To this end, a lidar wind detector has been developed.
As the idea of using lidar to determine wind characteristics has emerged, with it there has been a need for simple, effective and sensitive transmitters for transmitting a beam of light at an object to create backscattered light, and receivers for detecting the backscattered light. Lasers, which are the light sources for lidar, transmit beams which strike moving objects in the atmosphere (such as air-borne particles and molecules), wherein the backscattered light has undergone Doppler shifts due to the movement of the objects which can be used to determine the speed and direction of the object, and hence speed and direction of the wind. This backscattered, Doppler shifted, light is then gathered and directed into a receiver wherein it can be analyzed to determine the Doppler shift. There has been a recognized need for improved technology in gathering and directing the backscattered light.
It is advantageous for the lidar laser beam to sweep or scan along a path at an angle to the receiver; however, it has proven difficult not only to transmit, but to gather and direct the backscattered light into a receiving light sensitive element. A number of different approaches have been attempted, primarily using mirrors to vary the direction of the outgoing and incoming backscattered light. Generally, the mirrors are rotated and adjusted to direct the transmitted light beam so that it is backscattered by an object and to direct the backscattered light into the receiver; however, this approach has limitations.
One significant limitation to the use of mirrors arises from their size and mass. High resolution mirrors are required for lidar to provide accurate measurements, particularly where small objects are measured at great distances, as in the present invention. Generally, the resolution of a mirror improves as mirror size increases and high resolution mirrors are rather large. There are also problems in the manufacturing and deployment of large mirrors, particularly where the mirrors are used in satellites. First, there are material limitations, as large glass mirrors may crack or shatter, and large mirrors are awkward and cumbersome to handle. In addition there are production difficulties in producing mirrors within allowable tolerances of curvature and reflection. Finally, high resolution mirrors are further limited by their mass in deployment, as they add to the payload of the deployment vehicle.
Additional challenges are created with satellite deployment of large mirrors. The substantial mass of a high resolution mirror means the mirror has a large moment of inertia and large angular momentum when it is moving or turning. Further limitations arise from the motors and mechanical components required for operation of the mirrors because they must be of sufficient size to handle the inertia of the mirrors but must be very smooth and vibration free in operation as vibrations will adversely effect the resolution of the mirrors. Thus, not only are there technical challenges in producing movable mirrors with minimal mechanical vibration, Newtonian mechanics requires that the satellite include systems such as momentum wheels or gyroscopic stabilizers to compensate for the inertia of the mirrors and maintain satellite stability, otherwise the satellite may tumble. For this reason mirrors require significant amounts of fuel and energy to compensate for necessary mirror movement. Additionally, it is difficult to adjust, replace or repair a large mirror after satellite deployment should defects be discovered or develop. As can be seen, there are substantial costs and economic burdens associated with use of movable mirrors on satellites.
It would therefore be an advantage to have an apparatus or method of gathering backscatter and directing it into a receiver, which requires minimal mechanical elements, is small, easily and cost effectively manufactured, with a minimal mass, moment of inertia and angular momentum, while having substantially high optical clarity and resolution quality. It would also be an advantage if the device or method used components which are relatively easy and inexpensive to maintain, requiring little or no repair. It would be a further advantage to measure wind speed and direction at numerous and various points around earth using a movable vehicle such as a satellite. It would be an additional advantage to make measurements of wind characteristics in an efficient and cost effective manner.